The present invention relates to a casing structure of metal construction for the rotor blade area of axial flow compressor and turbine stages.
It is very important for the technical flow characteristics of compressor and turbine stages that the radial gap between the rotor blade tips and the casing be kept as small and as constant as possible. For this purpose, the casing structure should first be sufficiently stable in its dimensions and geometrically accurate. Thermal and mechanical influences should alter the geometry as little as possible. The mostly hot working gas should essentially act only on the inside wall of the structure, in order to minimize leakage losses through the structure. In variable operation, it is advantageous if the timing and magnitude of the changes, particularly thermally induced dimensional changes, of the casing structure are matched to those of the bladed rotor. Since mechanical contacts between the blade tips and the casing can rarely be avoided under certain loads, the inside wall of the casing structure should be deformable/flexible or abradable, at least on the blade tip side.
European Published Patent Application No. 0 728 258 relates to a shroud segment of a turbine, which together with segments of the same type forms the inside wall and a part of the connecting structure to the outside wall of a wall structure. Due to temperature differences between the inside and the cooled outside of the segments in operation, and to differences in the material properties of the base material and a coating generally applied, the segments have a tendency to alter their curvature. In order to prevent the segments locally shifting into the orbit of the blade tips, they are in places connected to the outer area of the casing structure by a special, hook-shaped geometry on leading and trailing edges, which in places permits a radial movement outwardly. Since the internal contour therefore often deviates from the circular with a tendency to form a polygon, it is difficult to maintain a defined gap. The sealing of the segments subject to a variable gap and play also makes for an expensive design.
European Published Patent Application No. 0 781 371 describes an arrangement for the dynamic control of blade tip play in gas turbines. The inside wall of the casing structure includes curved, circular arc-shaped segments, capable of moving radially outwardly and overlapping in a circumferential direction, the inward radial movement of which is limited by a peripheral casing structure, unilaterally retaining their leading and trailing edges in the manner of a hook. The segments are preloaded radially inwardly against a stop by mechanical spring elements or by gas pressure. The rotor blades include wedge faces at the tip, which, under high-speed rotation, generate a dynamic gas cushion, the pressure of which is intended to hold the wall segments at a small, defined distance from the blade tips. In so doing, an equilibrium must be established between internal gas pressure and external spring force, which keeps the segments in balance. Such a system appears very prone to malfunction, difficult to calculate and susceptible to oscillation. The retaining structure for the segments is exposed to the working gas and hence to potentially high thermal loads and also conducts a considerable amount of heat to the outside wall of the structure.
European Published Patent Application No. 0 616 113 relates to a gas turbine and a method for the fitting of a seal in the said gas turbine and describes the use of metal honeycombs as running-in surfaces for labyrinth seals. The honeycombs are brazed on one side to a sheet metal carrier, generally of closed annular geometry, its openings facing annular, bezel-like sealing tips. The deformation behavior of the thin, ductile, upright honeycomb walls speeds up any running-in process that may be necessary and protects the sealing tips. The open structure with a plurality of chambers increases the sealing effect by diverting and swirling the flow. In aircraft and boat construction, sandwich-type lightweight structures are used, in which a relatively thick, light core with a high proportion of void space, for example, a honeycomb, is covered with and connected to thin, high-strength closed walls. When such a structure flexes, the walls are primarily subjected to tensile or compressive loading in their plane, while the core transmits the forces, e.g., shear forces, from wall to wall. The walls may be of fiber-reinforced construction, bonded to the core and at least comparable in their thickness and mechanical characteristics.
It is an object of the present invention to provide a casing structure of metal construction for the rotor blade area of axial flow compressor and turbine stages, which provides a high dimensional and geometrical accuracy under changing operating conditions and temperatures, a high mechanical load-bearing capacity, good thermal insulation effect and a minimal operating fluid leakage through the structure, and which by particularly small, substantially non-varying gaps with the rotor blade tips permits a high stage efficiency and a high stage load.
The above and other beneficial objects of the present invention are achieved by providing a casing structure as described herein. The present invention therefore relates to the connecting structure arranged between the segmented inside wall and the closed, load-bearing outside wall, and in the fused integration of their materials. The connecting structure is designed as a light, filigree hollow-chamber structure, for example, as a honeycomb structure, occupying substantially all of the hollow space between the inside and outside wall and is connected to one or both walls by brazing. Due to the xe2x80x9cquasi sheet-likexe2x80x9d connection of the walls, it is possible to impress the geometrical accuracy of the load-bearing outside wall on the segmented inside wall in all operating conditions. Any warping or xe2x80x9cpolygonizationxe2x80x9d of the internal contour may consequently be prevented. Its xe2x80x9csheet-like characterxe2x80x9d makes the brazed connection ideal in terms of mechanical strength and durability and does not have any negative effect on the structure of the material. On the other hand, the filigree connecting structure is sufficiently elastic to permit thermal expansion/contraction of the inside wall segments in a circumferential direction without critical constraining forces. The connecting structure has a thermally insulating effect, which is due to its high proportion of void space and may also be influenced by the choice of material. The inside wall therefore absorbs the generally high temperature of the working gas, and the outside wall may be kept distinctly cooler, which is beneficial to its mechanical characteristics. The insulating effect may also improve the thermodynamic efficiency of the engine. The filigree connecting structure is substantially impermeable to gas in a circumferential and axial direction, so that additional sealing measures may be eliminated. The leakage through the few, small expansion joints of the inside wall is of no significance.